Automotive instrument clusters have limited room for displaying messages. With the advent of serial communication links in vehicles and more complex systems which must be monitored, the number of tell-tale warnings which accompany these systems has greatly increased to the point that there is insufficient available area for them. It has been proposed to use a small reconfigurable display which displays one message at a time. If more than one message is called for, a microprocessor will sequence the messages in time. Such a display is subject to a number of constraints. The area taken up in the cluster must be minimal and is not to be substantially larger than a single display area itself. It must operate over a wide temperature range, at least -40.degree. C. to +95.degree. C. Further the display must by as inexpensive as possible so that it is competitive. It is desirable that the message area be dead fronted when messages are not displayed.
It is desirable that a LCD be used for the display. Such displays require a driver which typically is an integrated circuit (IC). Most commercial LCD driving ICs are poorly suited for the application and tend to be over designed in some areas and under designed in others. For example, the LCD display for this application needs a display format of 14 pixels high by 90 pixels wide or 1260 pixels. Commercial LCD driving ICs are tuned for general applications and typically require two or more ICs to obtain the necessary pixel count. Commercial driver ICs use general output drivers which are much larger and more expensive than required for this application.
A small display can accommodate only a limited number of connections between the IC driver and the LCD. To handle the 1260 pixels with a few connections it is necessary to use a high rate of multiplexing. As multiplex rates increase the voltages used to operate the cell have to be controlled more precisely and have to be compensated over the temperature range. Due to the wide temperature range the maximum voltage varies greatly, say, from 6 to 12 volts. To assure correct voltage for every point in the range the temperature of the LCD cell has to be closely monitored, within a degree or so. The optimum voltage is also dependent on the liquid crystal material in the cell, and the LCD characteristics can vary from cell to cell or from batch to batch. The voltage must also be compensated for that variation.
In general purpose ICs, the number of messages which are to be displayed is not generally limited, so that such parts are designed to have each pixel state (1260 in this case) loaded into them by a microprocessor. However, downloading that much data by a microprocessor which is busy with other tasks comprises a severe software burden. In a tell-tale display there is a limited number of messages which must be displayed (typically 30 or less) and so it becomes possible and desirable to use another technique to avoid the downloading task.